Gene, chromosomal and genomic mutations

According to the change in the genetic material, three groups of mutations are distinguished: gene, chromosomal and genomic.

Gene mutations

Gene, or point (point), are called mutations resulting from changes in a gene, that is, the structure of the DNA molecule.
If replication is impaired, a change in the sequence of nucleotides in any part of DNA can occur. This could be:

• nucleotide replacement;
• nucleotide insert;
• nucleotide loss.

If a nucleotide substitution occurs, the result may be different. In some cases, this mutation does not lead to a change in the structure of the protein.

Example: consider the mutation GTT CCC GGT → GTT CCC GGT.
In the first triplet, thymine was replaced with cytosine. The triplets GTT and GTZ encode glutamic acid, so this mutation does not cause any changes in the protein structure: glu-gly-pro → glu-gly-pro.
In other cases, a nucleotide substitution can change the order of amino acids in a protein molecule and lead to phenotypic consequences.

Example: mutation GTT CCC GGT → GTG CCC GGT.
In the first triplet, thymine was replaced by guanine. The GTT triplet encodes glutamic acid, and the GTG triplet encodes histidine. This means that the primary structure of the protein changes: glu-gli-pro → gis-gli-pro. This can lead to phenotypic changes.

The addition or deletion of nucleotides leads to a shift in the reading frame in the ribosome and to a change in the amino acid sequence. A protein is synthesized that differs in its primary structure from the original. As a result, a serious change in the phenotype can occur.

Example: GTT ЦЦЦ ГГТ Т → ГТЦ ЦЦГ ГТТ.
The original DNA region encodes the amino acid sequence glu-gly-pro. After the loss of thymine in the first nucleotide, the amino acid sequence is different: lys-glu-glu. A mutagenic gene transmits new information to the site of synthesis, another protein is synthesized, which can lead to the emergence of a new trait.

Gene mutations lead to hereditary diseases such as phenylketonuria (metabolic disorder) and albinism (lack of normal pigmentation).

Chromosomal mutations

Chromosomal mutations are called mutations caused by a change in the structure of chromosomes.

This could be:

• loss (shortage) – loss of its terminal part by the chromosome;
• deletion – loss of a section of the middle part of the chromosome;
• duplication – duplication of a fragment of a chromosome;
• inversion – rotation of the chromosome section by 180 °;
• translocation – the transfer of a section of one chromosome to another.

Chromosomal mutations most often occur when cell division is abnormal. Their consequences for the body can be different. The most dangerous is loss and deletion, as information about a vital protein can be lost.

Violation of the structure of chromosomes in humans often leads to severe forms of mental retardation, blood diseases, and a decrease in the vitality of the body.

Example: The loss of a small part of chromosome 21 causes leukemia.
Chromosomal mutations can be detected with a microscope. Microscopy is used in the diagnosis of hereditary diseases.

Genomic mutations

Genomic mutations are called mutations caused by a change in the number of chromosomes in the karyotype of an organism.
Distinguish between polyploidy and aneuploidy (heteroploidy).
Polyploidy is a multiple increase in the haploid set of chromosomes.
It occurs when there is a violation of the divergence of chromosomes during mitosis or meiosis.

As a result, the chromosome set of the cell becomes triploid 3n, tetraploid 4n, hexaploid 6n, etc.

Polyploidy is widely used in plant breeding. Polyploid plants, as a rule, are characterized by more vigorous growth, higher productivity, and vitality. To obtain polyploid plants, colchicine is used, which destroys the spindle threads and leads to the formation of polyploid genomes.

Aneuploidy (heteroploidy) – a non-multiple change in the number of chromosomes 2n ± 1, 2n ± 2 …
This type of mutation can be caused by an excess or deficiency of one or more chromosomes. The cause of heteroploidy is the violation of the divergence of homologous chromosomes during meiosis. Both homologous chromosomes fall into one gamete, and none of them fall into the other. The fusion of such a gamete with a normal one leads to the formation of a zygote with a larger or smaller number of chromosomes compared to the original chromosome set.

The following forms of aneuploidy are distinguished:

• trisomy (2n + 1) – three chromosomes in one of the pairs (trisomy on the 21st pair of chromosomes in humans – Down’s syndrome);
• monosomy (2n − 1) – lack of one chromosome (monosomy on the X chromosome – Shereshevsky-Turner syndrome);
• nullisomy (2n − 2) – the absence of a pair of homologous chromosomes (lethal mutation).